/Engine/Source/Runtime/MeshDescription/Public/MeshDescriptionBase.h
Adds a new triangle to the mesh and returns its ID. This will also make an encapsulating polygon, and any missing edges.
[FTriangleID](API\Runtime\MeshDescription\FTriangleID) CreateTriangle ( [FPolygonGroupID](API\Runtime\MeshDescription\FPolygonGroupID) PolygonGroupID, const [TArray](API\Runtime\Core\Containers\TArray)< [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) > & VertexInstanceIDs, [TArray](API\Runtime\Core\Containers\TArray)< [FEdgeID](API\Runtime\MeshDescription\FEdgeID) > & NewEdgeIDs )